1. Field of the Invention
The present invention in general relates to a trunk coupling unit of a token ring. More particularly, the invention concerns a wiring or interconnection of relays incorporated in the trunk coupling unit.
2. Description of the Prior Art
As to the internal structure of the trunk coupling unit (hereinafter referred to as TCU in abbreviation) designed for interconnecting a token ring transmission path and individual stations, recommendation is made in "IEEE Draft Standard 802.5" (1984).
The TCUs are provided in correspondence to a plurality of individual stations and each serves for switching or controlling the data transfer between a ring-like transmission path and the associated station.
When the associated station is to be connected to the transmission path, the TCU operates to break the transmission path and connect one end of the broken transmission path to the input terminal of the associated station while connecting the other end of the broken transmission path to the output terminal of that station. On the other hand, when the associated station is to be disconnected from the transmission path, both the ends of the broken transmission path are again connected to each other and both the terminals of the station are interconnected. For accomplishing the connections mentioned above, the TCU includes a number of relay circuits.
The TCU of the hitherto known structure suffers a difficulty in that cross-talk takes place through the relay circuits provided internally of the TCU. For having a better understanding of the invention, the reasons for the occurrence of cross-talk in the conventional TCU will be explained below in some detail by referring to FIGS. 3 and 4 of the accompanying drawings.
FIG. 3 is a view for illustrating the occurrence of cross-talk upon connection of a station 1 to a transmission path 3 through an associated TCU 2. Referring to FIG. 3, each of relays 6, 7 and 8 has terminals a, b, and c, wherein the terminal a is selectively connected to either the terminal b or terminal c for connecting the station 1 to the transmission path 3 or disconnecting the former from the latter. A reference numeral 10 denotes a wiring conductor for establishing a connection between the terminals b of the relays 6 and 7, a numeral 11 denotes a wiring conductor for forming connection between the terminals c of the relays 6 and 7, a numeral 12 denotes a wiring conductor for realizing connection between the terminals b of the relays 8 and 9, and a numeral 13 denotes a wiring conductor for the connection between the terminals c of the relays 8 and 9.
In the arrangement shown in FIG. 3, an incoming signal A produced by a transmitter 5 of a preceding station 1 (not shown) is sent through its associated TCU to a receiver 4 of the illustrated station 1 through its relays 6 and 8. On the other hand, a signal B produced by the transmitter 5 of the station 1 is sent through the relays 7 and 9 to the transmission path leading to a TCU of a succeeding station (not shown). In that case, positive cross-talks C(+) of the signal B take place to the terminals c from the terminals b of the relays 6 and 7, respectively, to which the positive polarity of the signal is applied, both cross-talks being added together to be of a magnitude 2C(+) assuming that C(+) also represents the magnitude of each cross-talk. Additionally, negative cross-talks C(-), each having the same magnitude as C(+) and a polarity opposite to that of the latter, take place from the terminals b to the terminals c of the relays 8 and 9, respectively, to which the negative polarity of the signal is applied. Both the negative cross-talks are added together to be 2C(-). In this case, when the signal A becomes quiescent, the cross-talks 2C(+) and 2C(-) of opposite polarities are reproduced by the receiver 4 of the station 1, bringing about erroneous operation.
FIG. 4 is a view for illustrating the occurrence of cross-talk upon disconnection of the station 1 from the transmission path 3 through the associated TCU 1. In FIG. 4, like parts as those shown in FIG. 3 are denoted by like reference symbols.
Referring to FIG. 4, a signal D sent out from the transmitter 5 of the station 1 is fed to the receiver 4 thereof through the relays 7 and 9 and the relays 6 and 8. On the other hand, an incoming signal E derived from the TCU of the preceding stage through the incoming transmission path 3 is sent through the relays 6 and 8 and the relays 7 and 9 to the outgoing transmission path leading to the TCU of the succeeding stage. At that time, positive cross-talks F(+) of the signal E takes place from the terminals c of the relays 6 and 7 to the terminals b thereof, respectively, which are added together to be 2F(+). On the other hand, negative cross-talks F(-), each having the same magnitude as F(+) and having a polarity opposite to that of the latter, occur from the terminals c to the terminals b of the relays 8 and 9, a sum of which is then 2F(-). When the signal D becomes quiescent, both the cross-talks 2F(+) and 2F(-) are reproduced by the receiver 4 of the station 1 to bring about erroneous operation.